Chapter Five TWO PRESIDENTS... THREE GREAT SCENES THREE of the film's outstanding dramatic scenes revolved around two American Presidents. and two of the most delicate problems of casting in Hollywood history. The first shows the late President Roosevelt as he launched America on its two billion dollar atomic project., often termed the biggest gamble of all time. Mr. Roosevelt at the Little White House in Warm Springs Georgia, on the final day of his life. The third, as the re-enactment of the fateful meeting in Potsdam, Germany, when Mr. Truman gave his order for the use of the atom bomb against Japan.   Half-a-dozen skilled actors were considered for the role of Mr. Roosevelt before the final choice was made. Then Godfrey Tearle, who was born in New York but attained his greatest prominence in the English stage and screen, was flown in from London to Hollywood for the role. Although a brother of the late star f silent films, Conway Tearle, the actor never had previously visited the motion picture capital.   His startling resemblance to the famed American leader created an unusual stir wherever he went in Hollywood, just as it had in New York on his last appearance on l3roadway, in "The Flashing Stream" in 1939, and during the war when he entertained American troops in England and in Italy.   An interesting sidelight, and still another challenge to the filming of the picture, was the duplication of the President's office on an M.G.M. sound stage. To do a faithful job of it, the studio obtained permission from the White House to photograph various sections of the office. .even pictures on the walls were photographed, then blown up to their original size in .Hollywood and colored. In addition to White House permission, releases then had to be obtained from the copyright owners of each painting, which included several received by President Truman from the National Museum, and an oil painting of Mr. Roosevelt.   Recreating the same room as it was five years ago was more difficult, but it was achieved with the same fidelity , after careful scrutiny of photographs made of it at that time.   An unusual commentary on the habits of two Presidents was revealed in the picturing of the President's desk in the two sequences. For the scene of Mr. Truman, the desk was shown with scarcely more than a dozen objects neatly. placed on its surface. During the term of Mr. Roosevelt, the desk top contained ninety-eight articles. including six ashtrays, six boxes of matches and countless miniature elephants, dogs. horses, donkeys and other animals, all souvenirs sent by admirers.